Genetically determined differences in drug metabolism can influence the risk of idiosyncratic drug toxicity. Identification of these metabolic differences can provide an understanding of the pathogenesis of these toxic reactions and may contribute to their prevention. One well- characterized pharmacogenetic defect in humans involves a deficiency of cytosolic arylamine N-acetyltransferase (NAT). NAT deficiency has been associated in humans with increased risk of toxicity from drugs such as sulfonamides, procainamide, hydralazine, and isoniazid. The dog is the only naturally occurring model for sulfonamide hypersensitivity in humans. In addition, the dog has a poorly defined defect in NAT activity. Therefore, the specific aims of these studies are to characterize this defect in cytosolic arylamine N-acetylation in dogs, to compare the biochemical nature, molecular basis, and prevalence of this defect to that described in humans, and to characterize the role of an alternative microsomal acetylation pathway. Initially, Southern blotting and PCR amplification of canine genomic DNA will be use to screen for canineNAT genes, followed by Western blot analysis of canine hepatic cytosol for NAT protein. Northern blotting, DNA sequencing, and gene expression will be pursued if canine NAT gene(s) are identified. If no canine genes are found, carnivores related to dogs will also be screened for the presence of NAT genes to determine the evolutionary nature of gene loss. In vitro enzymatic assays in canine microsomes and HPLC detection will be used to characterize alternative acetylation pathways.